For decades, solar energy has been a leading technology in the search to replace fossil-fuel energy as a sustainable and clean energy source. However, cost and efficiency remain as major concerns. As the price of photovoltaics (PV) modules has dropped, the inverters for energy converters now count for more than 10% of the total cost of an entire PV system. Additionally, in a conventional PV system all solar modules link to a central inverter; therefore, the overall system performance can be brought down by an underperforming module in the array or individual solar cells blocked from sunlight.
To solve this problem, and to optimize each individual solar panel, the micro-inverter technology has been proposed to embed inverters into each photovoltaic module. However, at the present time, the cost of micro-inverters is still higher than the cost of centralized inverters in small systems (≤10 kW). Another major factor hindering the wide adoption of solar energy is a conflict between aesthetics and energy saving. Many consumers find the placement of arrays of solar panels on buildings to be unsightly.
Consequently ways of integrating photovoltaics into buildings, i.e. Building Integrated Photovoltaics (BIPV) have been proposed. BIPVs serve as building elements so that the appearance of houses won't be compromised due to the post-installation of solar panels. To address these two major issues, it is desired to have inverters which can be integrated with solar modules to form a transparent solar PV system on glass (PV-SOG). In a PV-SOG, in addition to inverters, all other components including solar cells and controller circuits can be designed using the same process. Therefore, not only the cost but also the unit size of a PV system is reduced. The transparency and scalability of the PV-SOG's exterior appearance make them attractive for application to BIPVs. Moreover, PV-SOG's possess better system reliability since every solar module is integrated with an individual inverter or inverter array.
The high voltage devices are the core of the inverters. Currently, state-of-the-art high voltage and high power devices use the popular SiC and GaN transistors. However, both of these wide bandgap semiconductors require epitaxial growth at high temperature on strictly selected single-crystal substrates, which excludes their application in SOGs. In contrast, thin film transistor (TFT) technology made at low temperature has become a promising candidate for PV-SOG. Several semiconductor materials have been tried out to make high voltage thin film transistor (HVTFT) devices. Amorphous Si and poly-Si HVTFTs have been studied since the 1980s. Although amorphous Si can provide high voltage up to 800 V; however, its poor performance (on/off ratio ˜104) due to its low mobility limits its application. Poly-Si HVTFTs show better driving capability, but their low blocking voltage and non-uniformity from grain boundaries make them inadequate to meet the requirements of PV-SOG. In addition, Si-based TFT technology suffers from the absorption of visible light, restricting its application for transparent electronics. Organic TFTs which offer low cost and low process temperature have been used in display and RFID technologies. A high-voltage organic thin-film transistor (HVOTFT) has been reported to show switch drain-to-source voltages higher than 300 V with a controlling voltage range from 0 to 20 V. However, its low mobility, poor long-term stability and degradation at higher temperatures exclude its application in PV-SOGs, which operate under sunlight radiation, and its lifetime, like the regular residential solar cells, is expected to be more than 25 years.
Since the report of Indium Gallium Zinc Oxide (IGZO) TFTs with high electrical performance at low process temperature, oxide semiconductor TFTs have emerged in many applications, especially in displays, and transparent electronics. In the area of HVTFT, it was reported that an IGZO HVTFT operated at above 100 V with an on/off current ratio of 107. Although this is beyond the regular operating voltage in TFTs, it is still not sufficient to be used in inverters for a solar PV system. Furthermore, it is desired to use indium-free materials due to the high cost of indium, especially in the case of large-area electronic systems such as solar cells. In addition, the toxicity of IGZO due to its high indium concentration is of considerable environmental concern.
A need thus exists to develop a new HVTFT on glass technology for the micro-inverters which can be integrated with solar modules to form the PV-SOG. It is also desirable to provide HVTFTs for various high voltage applications at low cost, such as self-powered smart glass.